The invention relates to a process for recovering tin from starting materials containing relatively low concentrations of tin.
Supply of tin is based worldwide upon rich tin concentrates because, to be economically desirable, occurrences of tin are based upon rich concentrates of workable ores. The efficiency of the traditional pyrometallurgical two-stage recovery process depends strongly on the tin concentration in the first concentrate runnings. Even where there are favourable cost relationships, the boundary-line of efficient processing must be seen as 40 to 50% Sn and also 6% Fe in the concentrate.
It is true that fuming processes have been developed and put to industrial use with the volatilization of SnS (S being a carrier in the starting material or subsequently added) or SnO or SnOS, for the enrichment of starting materials containing little to relatively low concentrations of tin. These fuming processes modify these tin starting materials up to a more concentrated tin material for subsequent further processing on traditional pyrometallurgical lines into metallic tin, but such fuming processes operate at high temperature (1000.degree. to 1400.degree. C.) with huge energy consumption and represent only an enrichment process. Chlorination processes, for reasons connected with equipment and corrosion engineering, have not been able to gain acceptance.
The advances made in preparation technology make it possible, in particular, efficiently to enrich, from oxide and oxide/sulphide starting materials or ores, concentrates having up to a 10 to 25% Sn content. Such starting materials can be added only to a limited extent to very rich Sn concentrates (much greater than 60%) for the traditional two-stage tin recovery, where an iron content of less than or equal to 6% also has has to be observed. A direct tin-recovery process from starting materials with about 10 to 30% Sn content does not yet exist and has not not been put to industrial use.
Starting materials of such low Sn concentration cannot be efficiently processed into metallic tin with presently conventional pyrometallurgical two-stage recovery process using:
1. partial metal reduction for tin recovery with slag formation having 8 to greater than 25% Sn, PA1 2. slag reduction with hard slag production containing 40 to 80 % Sn and 20 to 50% Fe and finishing slags containing less than 1% Sn, PA1 3. hard slag re-introduced into the first processing stage.
The task of the present invention is to provide a simple process with which it is possible to convert the first SnO.sub.2 /SnS runnings from a poor concentrate by a decomposition through melting into water-soluble compounds from which metallic tin can recovered electrolytically.
This problem is solved by having the decomposition by melting effected with potassium hydroxide under inert gas or in air.